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PlasticArm SoC places Arm Cortex-M0 on a flexible substrate


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An article published this week in Nature by Arm Research and PragmatIC, based in Cambridge, UK, revealed details of PlasticArm, a flexible Arm Cortex-M0-based system-on-a-chip (SoC) manufactured using high-performance transistors. thin film (TFT) on a flexible substrate.

Although not yet a commercial solution, it has great potential for integrating microprocessors, and therefore intelligence, into many other everyday products. A co-author of the paper, John Biggs, Distinguished Engineer at Arm Research, said of the development: “As ultra-low-cost microprocessors become commercially viable, all kinds of markets will open up with cases of. interesting uses such as smart sensors, smart labels and smart packaging. Products using these devices could contribute to sustainability by reducing food waste and promoting the circular economy through intelligent lifecycle tracking. Personally, I think the biggest impact could be in healthcare – this technology really lends itself to building smart disposable health monitoring systems that can be applied directly to the skin.

Flexible electronic devices, unlike conventional semiconductor devices, are built on alternative substrates such as paper, plastic, or metal foil. Using thin-film semiconductor materials such as organics, metal oxides, or amorphous silicon, they offer properties not available from silicon, including thinness, conformability, and low manufacturing costs. TFTs can be fabricated on flexible substrates at a significantly lower processing cost than metal-oxide semiconductor field effect transistors (MOSFETs) fabricated on crystalline silicon wafers.

Catherine ramsdale

EE Times spoke to one of the article’s other co-authors, Catherine Ramsdale, senior vice president of technology at PragmatIC. She explained, “It’s basically a proof of concept, showing what you can do and the complexity that can be achieved. We’ve been working with Arm since 2013, and now we can say that the technology is at a certain level of maturity where the number of doors is achievable and the ecosystem is in place.

Arm Research and PragmatIC began exploring the feasibility of a flexible Arm-based processor in 2013, starting with building prototype circuits including ring oscillators, counters, and shift register matrices. While some flexible components, such as sensors, memories, light emitting diodes, etc. have been prototyped, so far a flexible microprocessor has been a major obstacle to achieving fully flexible electronics.

When the PragmatIC FlexLogIC manufacturing system became available a few years later, along with the advancements in the PlasticArmPit project, which uses the same cell library, same tool flow, and same process technology, all the pieces of the puzzle fell into place, according to Arm. The teams at both companies decided it was time to try again, and on October 27, 2020, they declared that the world’s first fully functional silicon-free Arm processor, PlasticArm, was produced.

Although PlasticArm is an ultra-minimalist Cortex-M0-based SoC, with only 128 bytes of RAM and 456 bytes of ROM, it is twelve times more complex than previous cutting-edge flexible electronics. Details of this development are presented in the document, “A native flexible 32-bit arm microprocessor“.

We asked Ramsdale about some of the challenges in the development of PlasticArm. “A lot of the initial work was making sure we spoke the same language – for example, Arm was talking about larger circuits while we would be referring to smaller circuits. We had to make sure that the co-optimization of the design process was successful. We had to make sure that the technology used standard design tools that a silicon designer would use. “

“Therefore, we provided Arm with a PDK, which they implemented in their tools. Our part of this puzzle is to enable and ensure accurate capture of all the design rules of our process and ensure repeatability with the FlexLogIC enclosure. She added: “Where we are today is validation of how far we have come with FlexLogIC: to produce something at this scale, and not just for RFID chips. “

PlaticArm elements

There are three main components of the native flexible microprocessor: a 32-bit processor, a 32-bit processor containing a processor and processor peripherals, and a system on a chip (SoC) containing the processor, memories, and bus interfaces. all made with metal oxide TFTs on a flexible substrate. The natively flexible 32-bit processor is derived from the Arm Cortex-M0 + processor supporting the Armv6-M architecture (a rich set of over 80 instructions) and the existing toolchain for software development (compilers, debuggers, editors links, integrated development environments).

PlasticArm architecture and comparison with the Arm Cortex-M0 processor
The SoC architecture, showing the internal structure, processor, and system peripherals (left). And on the right: the characteristics of the CPU used in PlasticARM compared to those of the CPU Arm Cortex-M0 +. (Source: Nature)

The entire natively flexible SoC, called PlasticARM, is able to run programs from its internal memory. It contains 18,334 NAND equivalent gates, making it the most complex FlexIC (at least 12 times more complex than previous integrated circuits) ever built with metal oxide TFTs on a flexible substrate.

This processor fully supports the Armv6-M instruction set architecture, which means that the code generated for a Cortex-M0 + processor will also execute on the processor derived from it. The processor includes the CPU and a nested vector interrupt controller (NVIC) tightly coupled to the CPU, handling interrupts from external devices.

The rest of the SoC consists of memories (ROM / RAM), the AHB-LITE interconnect matrix (a subset of the Advanced High Performance Bus (AHB) specification), and interface logic to connect memories to the processor, and finally a bus interface which is used to control two general purpose input / output (GPIO) pins for off-chip communication. The ROM contains 456 bytes of system code and test programs, and has been implemented as combinatorial logic. The 128 bytes of RAM was implemented as a lock-based registry file and is primarily used as a stack.

Layout and micrograph of the PlasticArm matrix
Layout of the PlasticArm matrix (left) and matrix micrograph, showing dimensions (right). (Source: Nature)

PlasticARM is implemented with the 0.8 µm process of PragmatIC using standard chip implementation tools, along with a process design kit (PDK), standard cell library, and simulations of devices / circuits in order to implement the PlasticARM FlexIC. The flexible SoC is manufactured using the commercial “fab-in-a-box” manufacturing line, FlexLogIC.

The process uses indium gallium zinc oxide (IGZO) n-type metal oxide TFT technology and generates the FlexIC design on a 200mm diameter polyimide wafer. IGZO TFT circuits are produced using conventional semiconductor processing equipment adapted and configured to produce devices on a flexible substrate (polyimide) with a thickness of less than 30 µm. They have a channel length of 0.8 μm and a minimum supply voltage of 3 V.

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